This invention relates to a method of preparing a composite sheet by mixing in an aqueous media particulate material and polytetrafluoroethylene (hereinafter referred to as PTFE) particles which are subsequently fibrillated therein to form a unitary matrix of entangled PTFE fibrils containing the particulate material. The invention relates further to a high tensile strength composite sheet formed by this method.
Because of its unique hydrophobic surface character, high temperature stability and inertness, PTFE is used to great advantage in many situations. Microporous sheets of PTFE are used advantageously as electrolytic diaphragms for oxygen generators, in fuel cells, for filtering media, as battery separators and in other areas which require a relatively inert hydrophobic porous member.
It has been long recognized that, under shear stresses, small particles of PTFE in the presence of a suitable liquid lubricant will form fibrils or fibers of a microscopic size. Early processing involved adding particulate PTFE to white oil and extruding the mixture to obtain fibrillation. Later processes involved employing other hydrocarbon liquid lubricants with PTFE powders. Some processes require sintering of the fibrillated product to produce a final integrated porous article. Certain other processes involved employing a removable material to obtain pores or voids in the composite. This removable material is added in the process and removed after fibrillation and formation of the article. Examples of such removable materials include polymethylmethacrylate which may be dissolved from the sheet by suitable organic solvent or heat-fugitive materials which are removed by heating, e.g., during sintering.
Microporous PTFE sheets are known in the art, as exemplified by Goldsmith (U.S. Pat. No. 3,281,511) which discloses the production of a microporous sheet material which is produced by mixing PTFE powder with a carrier and a petroleum distillate such as "Stoddard" solvent (a flammable material) to produce a sheet. Goldsmith requires a readily removable filler such as ammonium carbonate to, upon removal, produce porosity in the resultant sheet. Stand (U.S. Pat. No. 3,242,246) calenders a mixture of PTFE particles and trifluorovinyl chloride to produce a homogeneous mass which is dipped into a solvent which dissolves the trifluorovinyl chloride and then sinters the resultant structure to obtain a porous article. Forgione et al (U.S. Pat. No. 3,766,013) blends a water-soluble carrier polymer (for enzyme material) with PTFE powder and an inert, solid, water-soluble component material and subjects the mixture to high shear to fibrillate the PTFE.
Vallance (U.S. Pat. No. 3,890,417) discloses a method of making porous diaphragms comprising preparing an aqueous dispersion of PTFE and a solid removable particulate filler, thickening the dispersion to effect agglomeration of the solid PTFE particles, forming the thickened dispersion into a sheet, biaxially calendering the sheet and removing the solid particulate filler material. Vallance's articles appear to require additional support means such as embedded plastic scrims to make them self-supporting.